1. Field of the Invention
This invention relates to an apparatus and a method for multiplexing signals of different frequency bands, and, in particular, to a low cost signal multiplexer and method that combines a national standard television code (xe2x80x9cNTSCxe2x80x9d) signal with a digital television (xe2x80x9cDTVxe2x80x9d) signal.
2. Description of the Prior Art
The introduction of digital television service has resulted in a need for a station to broadcast the television picture in the NTSC format for reception by television sets that are not equipped for digital reception and in DTV format for reception by television sets that are equipped for digital reception. These stations have been allocated the next higher channel for DTV service. That is, the NTSC signal is broadcast on channel N and the DTV signal is broadcast on channel N+1, channels N and N+1 being adjacent channels. There is a need for these stations to combine the NTSC and DTV signals into a composite signal for transmission on a common transmission line and antenna.
Prior art multiplexers include a branch combiner, a star combiner and a manifold combiner. These signal combiners have a serious disadvantage in that they do not present a constant impedance to the inputs that receive the signals. That is, out of band energy is reflected back to the inputs.
Another prior art multiplexer presents a constant impedance to the input. An example of this style of multiplexer is shown in U.S. Pat. No. 4,029,902. As shown in this patent, this style of multiplexer typically has a pair of matched filters, two hybrids and a load for each signal channel. The filtered output of each channel is fed as an input to the next channel and so on. Although reflected energy to the channel inputs is nil, the cost is high because 2N hybrids are required, where N is the number of signals or channels to be multiplexed. Moreover, the bandwidth is limited to the short circuit VSWR (voltage standing wave ratio) of the output hybrid. The short circuit VSWR is dependent on the amplitude balance of the output hybrid of each channel. In coax hybrids, the amplitude balance is controlled in the design of the hybrid by the use of multiple quarter wave coupled sections, which are power limited. In wave guide hybrids, which can take more power, the amplitude balance is limited by the inability to produce multiple quarter wave coupled sections. Consequently, when the bandwidth causes the amplitude ripple to exceed 0.085 dB, the short circuit VSWR exceeds 1.04:1. The sum of the VSWR of the individual channels quickly accumulates until the system exceeds a VSWR of 1.10:1 that is required by the transmitters.
There is a need for a signal multiplexer that is capable of combining an NTSC signal with a DTV signal for transmission on a common transmission line and antenna that is not bandwidth limited by the quality or power capacity of components, such as hybrid couplers.
A multiplexer according to the invention includes a first hybrid coupler and a first pair of band pass. filters to convert the NTSC signal into a first pair of filtered quadrature phase signals. A second hybrid coupler and a second pair of band pass filter converts the DTV signal into a second pair of filtered quadrature phase signals. A signal combiner combines the first and second pairs of filtered quadrature phase signals to produce a pair of multiplexed quadrature phase signals. A notch filter is connected to the signal combiner and is tuned to the aural frequency band of NTSC signal. A third hybrid coupler converts the pair of multiplexed quadrature phase signals into a single multiplexed signal.
A video component of the NTSC signal is converted by the first hybrid coupler to produce the first pair of quadrature phase signals. The third hybrid coupler converts an aural component of the NTSC signal into a pair of aural quadrature phase signals that are reflected by the signal combiner. The third hybrid coupler combines the reflected pair of aural quadrature phase signals and the pair of multiplexed quadrature phase signals into the single multiplexed signal.